Leek seedlings not progressing

sy6owl

Try as I may, I cannot get leek seedlings to reach the 'pencil thickness' referred to in 'How to...' articles on growing leeks. Year after year whether they're sown outdoors or as this year in a pot, the seedlings refuse to grow to the 'pencil' thickness . Neither do they grow very tall. The seedlings in the 9" deep pot in the photo are grown from seeds (Lyon Prizetaker) I sowed in the last week of March in peat-free multipurpose compost topped with a 1 inch layer of seed John Innes compost. I've kept them watered and have recently foliar sprayed them with liquid seaweed but to no avail. Here we are 2 months later and its happening again - they've stalled and have shown no signs of getting any bigger over the last 3 weeks. What is happening? Where am I going wrong?